during the 11- to 14-week measurement period is thus indicative of congenital lymphedema. Nuchal fold thickness Nuchal translucency testing is distinctly different from and should not be confused with nuchal thickness testing. At the end of the second trimester (26 weeks), the nuchal translucency can no longer be seen and instead the nuchal fold thickness is measured between 16 and 24 weeks gestation. The fold is more focal and at the level of the posterior fossa. This measurement has a higher threshold of normal, although the implications of increased thickness are similar to those of translucency. The nuchal fold thickness is considered normal if under 5mm between 16 and 18 weeks gestation and under 6mm between 18 and 24 weeks gestation. An increased thickness corresponds to increased risk for aneuploidy and other fetal abnormalities. History The nuchal scan first came into widespread use in 2003. See also Prenatal testing which further discusses the reasons for prenatal screening and the ethics of such testing. Congenital lymphedema List of obstetric topics Obstetric ultrasonography References Obstetrical procedures Tests during pregnancy Medical ultrasonography